In order to meet government mandated exhaust gas emission standards, the exhaust gases of an automotive internal combustion engine must be treated before emission into the atmosphere. Exhaust gas streams are typically passed through a device containing a catalytic element to remove undesirable gaseous emission components such as unburned hydrocarbons, carbon monoxide and nitrogen oxides. The removal of NOX components is of particular concern, and can be accomplished using a device employing elements containing a NOX adsorber catalyst. However, NOX adsorber catalysts are sensitive to sulfur poisoning and must be periodically desulfurized, otherwise their performance gradually decreases.
In addition to the gaseous components, exhaust gas streams also contain particulate matter such as carbon-containing particles or soot. A particulate filter, also commonly used with compression ignition engines, is used to prevent the carbon particles or soot from exiting the tailpipe. The particulate filter is typically a stand-alone device separate and distinct from devices employing catalytic elements for removing undesirable NOx gaseous components. Carbon particles are trapped in the particulate filter and continuously and/or periodically burned so that the filter is regenerated and able to again trap the carbon particles.
A catalytic particulate filter comprises a substrate and a catalyst composition typically formed from an inorganic precursor and a catalytic metal. The catalytic particulate filter catalyzes the removal of pollutants from exhaust gases as well as removes particulate from the exhaust gas stream. The catalyst composition is optimized for periodically regenerating the filter.
If the catalytic element and particulate filter (or catalytic particulate filter) are separate elements, they require a large installation space in the exhaust line, which is usually not available, as well as a high installation cost. Moreover, since the devices are serially mounted in the exhaust line, temperature losses inherent to heat capacity would affect the regeneration or desulfurization capabilities of the subsequent device since the regeneration or desulfurization of the respective device is thermally controlled.